About 150 Kurdish students were kidnapped last week by the radical Islamist group the Islamic State of Iraq and Syria, and they remain in a prison in the ISIS-held town of Manjib in northern Syria, adding to what's quickly becoming one of the worst mass kidnapping incidents of the Syrian civil war.

According to local journalists, activists and Kurdish news accounts, the students were taken in a series of kidnappings as they travelled from their villages to the city of Aleppo for their final exams. Despite the civil war wracking the country, many schools have continued to function, even in rebel-held areas.

At least 193 Kurds abducted in recent days are being held by ISIS, including the students, according to the Syrian Observatory for Human Rights, a Britain-based organisation that monitors violence in Syria. Kurdish assessments place the number of missing as much higher, saying hundreds of Kurdish residents have disappeared.

The situation provides a window into the incredible complexity of the Syrian conflict. ISIS controls portions of the countryside outside Aleppo, Syria's largest city. Aleppo itself is divided between the government and other rebel groups who've been fighting ISIS since January over its extreme al-Qaida-inspired ideology and its brutal tactics.

Meanwhile, ethnic Kurds in the area, especially militias associated with the Kurdistan Workers Party, have been battling to keep ISIS from northern Syria's Kurdish areas. ISIS, for its part, has been attacking Kurds - who are ethnically and culturally distinct from the Arabs who are the majority in the rest of Syria - in part because the Kurdistan Workers Party hews to a leftist ideology that's anathema to ISIS's conservative beliefs.

Mutlu Civiroglu, a Kurdish affairs analyst based in southern Turkey, said in an e-mail that there had been suggestions that ISIS had sent the abducted students to a workshop in Islamic law and they'd be released after the course was completed.

Others, however, think they're being held to exchange for members of ISIS who are being detained by a Kurdish militia, known by its initials YPG, that's loyal to the Kurdistan Workers Party.

"They claim this Shariah education is all that is necessary, but I think this is giving them time to make demands for the return of ISIS prisoners we have captured or perhaps for something worse," said a YPG security official reached via Skype. He noted that in addition to the missing students, ISIS had kidnapped scores of residents of villages in the area, "including old men and young boys."

The official said ISIS had released female students and some younger children almost immediately because, he suspects, they couldn't handle all the people they'd grabbed.

"They'd taken too many students and other villagers to manage holding them," he said. He asked not to be identified for security reasons.

"Relatives are terrified," he said, noting that ISIS' reputation for brutality toward captives has been well documented in dozens of internet videos that have shown captives tortured, executed and even beheaded.